Spark plug having porcelain insulator protected by plasma-jet deposited refractory coating



Oct. 11, 1966 w. E. HAUTH, JR 3,278,785

SPARK PLUG HAVING PORCELAIN INSULATOR PROTECTED BY PLASMA-JET DEPOSITED REFRACTORY COATING Filed Dec. 12. 1960 I I .3 O fl a; A INVENTOR. //////a/ad%a //& /2

ATTO R NEY United States Patent 3,278,785 SPARK PLUG HAVING PORCELAIN INSULATOR PROTECTED BY PLASMA-JET DEPOSITED RE- FRACTORY CGATING Willard E. Hauth, Jr., Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 12, 1960, Ser. No. 75,397 4 Claims. (Cl. 313-137) This invention pertains to spark plugs, and particularly to an improved spark plug insulator.

It is recognized that the firing tips of spark plug insulators are susceptible to corrosion attack under severe engine operating conditions due to accumulation of lead salts. This invention relates to means for inhibiting corrision attack on the firing tips of spark plug insulators by coating portions of the insulator with a fused refractory material that is resistant to penetration of lead compounds. Accordingly, among my objects are the provision of an improved spark plug insulator; the further provision of a spark plug insulator having a corrosion inhibiting coating; the further provision of a spark plug including an insulator having a firing tip coated with fused refractory material; and the still further provision of a method of applying a corrosion inhibiting coating to spar-k plug insulators.

The aforementioned and other objects are accomplished in the present invention by coating the firing tip of a spark plug insulator with a fused refractory material by means of plasma torch spray apparatus. Specifically, it is known that aircraft spark plugs which are used in high performance piston type engines experience erosion at the firing tip of the insulator due to accumulation of lead salts from the fuel. By coating the firing tip of the insulator with a fused refractory material that is resistant to penetration of lead compounds and is an electrical insulator, the coating thickness being in the range from .0005 to .010 inch, tests have revealed that insulator tips so coated are able to resist lead attack.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing which is a side view of the improved spark plug partly in section and partly in elevation, and wherein a preferred embodiment of my invention is clearly shown.

With particular reference to the drawing, a spark plug is shown comprising a metal shell having a ceramic insulator !12 coaxially secured therein. The ceramic insulator 12 may be of the alumina-silica type having a composition as disclosed in Schwartzwalder et al. Patent 2,760,875. Thus it will be appreciated that the insulator 12 is of the sintered glass bonded corundum type which is fired in the temperature range between 2700" F. and 3200 F. to form a dense, nonporous, sintered body.

The ceramic insulator 12 has a body portion 1 6 and a tapered tip portion 18, the body portion 16 being suitably held in snug relation with the shell 10. A lower gasket assembly 20 is interposed between juxtaposed shoulders of the shell 10 and the insulator 12. The ceramic insulator 12 is formed with a centerbore within which a conventional centerwire assembly 22 is secured. The centerwire assembly 22 protrudes from the lower end of the insulator 12 and constitutes a firing tip which forms a spark gap with a ground electrode 24 suitably attached to the metal shell 10.

The firing tip #1 8 of the insulator 12 has a frusto-conical recess 26 therein, and thus includes an inner wall portion which is radially spaced from the centerwire assembly, or center electrode, 22. This exposed inner wall of the insulator tip, and the outer wall of the insulator tip, are susceptible to erosion or corrosion due to the accumulation of lead salts. In order to inhibit this corrision, which seriously limits the useful life of a spark plug, the inner and outer walls of the firing tip of the insulator 12 are coated with a fused refractory material 14. The coating 14 may be of any suitable refractory material which is capable of resisting penetration of lead compounds, and thus may be selected from the group consisting of substantially pure aluminum oxide, magnesium oxide, beryllium oxide, rare earth oxides or refractory carbides and nitrides. Pure aluminum oxide is the preferred coating material since tests have revealed that when the insulator tips are coated with fused pure aluminum oxide, erosion and lead penetration are nonexistent.

As alluded to hereinbefore, the coating of fused refractory material is applied by the use of plasma torch spray apparatus. Plasma torch spraying or plasma jet spraying involves a device referred to as an arc plasma generator which comprises means for producing a very high temperature stream of an ionized non-oxidizing gas (a plasma) moving at a high velocity. In the present instance the non-oxidizing gas may have an analysis by volume of ten percent hydrogen and ninety percent nitrogen. Particles of powdered refractory material which are introduced into this stream are accelerated therewith and at least melted therein. For example, when pure aluminum oxide particles are introduced into the stream, it has been found that these particles will melt at approximately 10,000 F. The small refractory particles are accelerated away from the generator by the plasma stream emitted therefrom. This refractory-containing stream is directed toward the firing tip of the insulator where the refractory material is impacted so as to produce a fused coating of pure refractory material within the thickness range of from .0005 to .010 inch. A typical plasma-jet spraying device is shown in United States Patent 2,922,869. However, it is to be understood that this device is only exemplary and is not to be construed by way of limitation since any suitable plasma-jet spraying apparatus can be used to practice the present invention.

No significant advantage is obtained by applying a coating having a thickness greater than .010 inch. In addition, since it is only the firing tips of the insulators which are susceptible to penetration of lead compounds, and hence erosion, it is only necessary to coat the external surface of the insulator tip throughout approximately half its length between the lower gasket seat and the firing tip.

It will be understood that while the invention has been described specifically with reference to a preferred embodiment thereof, changes and modifications may be made all within the full and intended scope of the claims which follow.

I claim:

1. A spark plug comprising a ceramic insulator having an unglazted firing tip portion and a centerbore, a center electrode coaxially secured within the centerbore of said insulator and protruding from the unglazed firing tip portion thereof, and a thin corrosion inhibiting coating, 0.0005 inch to 0.010 inch thick, of a fused plasma jet sprayed ceramic refractory material covering the firing tip portion of said insulator.

2. A spark plug as set forth in claim 1 wherein a fused lasma jet sprayed ceramic refractory material is selected from the group consisting of aluminum oxide, magnesium oxide, beryllium oxide, the rare earth oxides and the refractory carbides and nitrides.

3. A spark plug as set forth in claim 1 wherein the fused plasma jet sprayed ceramic refractory material is aluminum oxide.

4. A spark plug comprising a ceramic insulator having an unglazed firing tip portion and a centerbore, said insulator having an enlarged recess communicating with the centerbore in the unglazed firing tip portion, a center 3 electrode coaxially disposed within the centerbore, and a thin corrosion inhibiting coating, 0.0005 inch to 0.010 inch thick, of a fused plasma jet sprayed ceramic refractory material covering the Walls of said recess and the outer wall of the unglazed firing tip portion of the insulator.

References (Iited by the Examiner UNITED STATES PATENTS Re. 15,585 4/ 1923 Sauereisen.

1,980,182 11/1934 Brewster 117-125 McDougal 313--141.1 Furth ,117125 X Schwartzwalder 313141 Aura-nd et a1. 313-141 Yenni et a1. 21976 Sullivan et a1. 21976 HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, ARTHUR GAUSS, Examiners. 10 S. CHATMON, JR., Assistant Examiner. 

1. A SPARK PLUG COMPRISING A CERAMIC INSULATOR HAVING AN UNGLAZED FIRING TIP PORTION AND A CENTERBORE, A CENTER ELECTRODE COAXIALLY SECURED WITHIN THE CENTERBORE, A CENTER INSULATOR AND PROTRUDING FROM THE UNGLAZED FIRING TIP PORTION THEREOF, AND A THIN CORROSION INHIBITING COATING, 0.0005 INCH TO 0.010 INCH THICK, OF A FUSED PLASMA JET 